1. Field of the Invention
The invention relates to threaded connections for two metal pipes with a tapered thread and with trapezoidal threads.
Such connections are known, in particular for strings of casing pipes or production tubing or drillpipe strings for hydrocarbon wells.
2. Discussion of the Background
In the remainder of the present document, the term xe2x80x9cthreaded connection for two metal pipesxe2x80x9d will encompass both an integral connection between two long pipes and a connection between a first, long, pipe and a second, short, pipe such as a coupling.
The American Petroleum Institute (API) defines:
in specification API 5CT, metal pipes and threaded metal pipe connections for production and for casing hydrocarbon wells;
and in specification API 5B, standard tapered thread forms for such connections.
The threads of such API thread connections can be trapezoidal and then comprise, on each of the male and female elements, a thread root, a thread crest and two flanks, namely a load flank and a stabbing flank.
The thread roots and thread crests are normally parallel to the taper of the thread.
The load flanks are so termed because on bearing against each other when the connection is subjected to tensile forces, for example due to the weight of the pipes, they enable the connection to tolerate such tensile forces. The load flanks are located on the threads opposite the stabbing flanks.
When making up such an API connection, depending on the taper of the threads, at a given moment corresponding to a given relative position of the male and female elements, the thread roots of one of the elements come into contact with the thread crests of the other element.
If screwing of the male element is continued into the female element beyond that position, the male threads start to interfere radially with the female elements, which leads to an expansion of the female element and a contraction in the male element; such an interference must be limited so as not to develop excessive stresses or deformation.
The diametrical interference between the mated points of two surfaces of revolution which radially interfere is generally defined as the difference in the diameter of the half cross section of the surfaces at those points, the difference being measured before connection and taken to be positive when the two surfaces, once connected, exert a contact pressure between the mated points.
To limit such stresses or deformations, an annular bearing surface which is orientated substantially transverse with respect to the axis of the connection can be provided on each of the male and female elements, the bearing surfaces being positioned such that they come into abutment with each other at a given moment during makeup thus precisely defining a makeup completion position.
The position at the end of connection makeup is, for example, determined by the torque required to arrive at that position.
The use of abutting bearing surfaces to position the connection has other advantages:
placing the load flanks of the connection threads under tension which are then ready to tolerate the tensile stresses to which the connection is subjected during service;
precise positioning of the male and female elements thus guaranteeing, when each of the male and female elements of the connection comprises a sealing surface which radially interferes with that located on the mating element, a high metal-metal contact pressure between the surfaces with no risk of plastification thereof;
reduced risk of accidental breakout because of the makeup torque which has to be overcome before being able to break out the connection, this torque being well defined and always above a minimum value.
European patent EP-A-0 488 912 describes such a connection with tapered threads screwed one into the other, a pair of radially interfering metal-metal sealing surfaces and a pair of abutting bearing surfaces, namely a concave tapered surface at the end of the male element and a convex annular surface forming an internal shoulder on the female element.
Such a threaded connection can be made up with high nominal makeup torques which can be up to 34 kN.m (25000 lbf.ft), for example, which is sufficient in the majority of cases.
However, it may be necessary to make up the connection with even higher torques, in particular for casing pipes for multiple deviated wells or horizontal wells enabling a wide zone to be exploited from a single site.
The use of techniques for rotating the string comprising drillpipes at their end (drilling liner) also permits better cementing of horizontal wells but necessitates pipe connections made up with torques which are higher than the rotational torque of the string if it is desired to prevent the threaded elements from rotating with respect to each other when the string is rotated, which rotation between elements can modify the characteristics of use of the connections, in particular their sealing properties.
Table 1 below gives an idea of the desired makeup torques for such applications.
The abutting bearing surfaces can only tolerate such torques without deterioration if the radial width of the abutment surfaces is increased, but then much thicker pipes have to be used which may be incompatible with service requirements.
Thus other means have to be used than abutting bearing surfaces to absorb high makeup torques.
International patent application WO 94/29627 describes a threaded connection with a tapered thread and trapezoidal threads known as wedge threads in the general form of a dovetail and more particularly a half dovetail.
Such threads are known as wedge threads or threads with a variable width since the width of the male and female threads varies from one end of the thread to the other in a manner which is coordinated between the male and female threads.
Such threads are termed xe2x80x9chalf dovetailxe2x80x9d since they overhang the thread roots on one side only, either on the load flank side, or on the stabbing flank side, and because the angle between the load flank and the normal to the connection axis and that between the stabbing flank and said normal is such that the thread width is higher at the crest than at the root.
When the male element is engaged in the female element in accordance with WO 94/29627, the narrowest thread crests face the widest thread roots and there is a large axial clearance between the mating flanks of the threads.
As the male element is screwed into the female element, the axial clearance reduces to a position where the two male flanks come into contact with their female mates.
Beyond that position, the female flanks interfere with the male flanks and there ensues a very rapid increase in the curve of the makeup torque as a function of rotation.
Such a connection in accordance with WO 94/25627 can certainly tolerate a high makeup torque due to the developed surface of the threads but it suffers from a number of important disadvantages.
Firstly, variable width wedge threads are expensive to machine and difficult to inspect.
Further, the acute angles of dovetails or half dovetails disposed on the load flank side and/or on the stabbing flank side, constitute sharp angles which are sensitive to cuts and flash from such cuts are deleterious to the function of the connection.
Such sharp angles also notch the thread roots and as a result the threads are more fragile during use.
The present invention seeks to provide a threaded connection which can be made up to a high makeup torque T which is free of such disadvantages and in particular a threaded connection which is economical to machine and which can be readily manipulated on-site.
We have also sought to provide a threaded connection whereby the desired makeup torque is obtained after considerable rotation, for example of the order of one turn, or more.
We have also ensured that in certain configurations, the slope of the makeup torquexe2x80x94rotation curve is reduced from a given torque, resulting in a self-limiting characteristic for the makeup torque.
We have also sought to provide a connection which is particularly tight to internal and/or external fluids, even after a number of makeup-breakout operations.
The threaded connection between two metal pipes of the present invention comprises a male element at the end of a first pipe screwed into a female element at the end of a second pipe.
The male element has an external tapered male thread with trapezoidal threads where the thread width at the thread crests is less than the thread width at the thread root.
The female element comprises an internal tapered female thread with trapezoidal threads with a form which mates with that of the male thread.
The term xe2x80x9cfemale thread mating with that of the threadxe2x80x9d here means that the taper and pitch of the female thread are substantially identical to those of the male thread and that the thread form of the female threads is substantially identical to that of the male threads, the inclination of the load flanks and stabbing flanks of the female threads to the connection axis being in particular identical to that of the corresponding flanks on the male elements, the width of the female thread crests being less than that of their root as with the male threads. Clearly, the form of the male thread then reciprocally mates with that of the female thread.
The width of the thread crests on each of the male and female threads is higher than the width of the space between the roots of the mating threads.
The male element is positioned by screwing into the female element to a relative position of these two elements located beyond that where, during makeup, the two male thread flanks come into contact with the two female thread flanks, so as to induce an axial interference fit of the male threads by the female threads, and vice versa.
Depending on the mating form of the male and female trapezoidal threads used and because the thread width at the crest is lower than the thread width at the root, the male and female threads penetrate radially and wedge into the mating hollows by a wedging effect as the axial progression of the threads occurs during makeup and thus, beyond the position of contact of the two mating flanks, induce an axial interference fit of the male threads by the female threads and vice versa.
This interference fit over all of the surface of the flanks results in the possibility of absorbing a high degree of makeup torque T in the threads.
The features of such threads are such that they can be made cheaply, they are easy to inspect and are not fragile in use.
EP-A-0 454 147 describes a threaded connection with a tapered thread and trapezoidal threads where the width of the female thread crests is higher than that of the male thread roots and in which when connection makeup is complete the two flanks of the thread of one element are in contact with those of the mating element, at least over a portion of the thread.
However, in EP-A-0 454 147, only simple contact has been aimed at, even only partial contact, between the mating flanks so that, when the connection is to be subjected to compression stresses after having been subjected to tensile stresses, there is no re-positioning of the stabbing flanks due to an axial clearance which pre-exists with the latter, which re-positioning can cause plastification of the metal, in particular the sealing surfaces, and can thus cause a subsequent risk of leakage when the connection is again subjected to tensile stresses. To obtain such a simple contact of the thread flanks when connection makeup is complete, EP-A-454 147 has to use means for positioning the elements, namely a transverse bearing surface on each element, each bearing surface being able to abut against that of the mating element. The mating male and female stabbing flanks are disposed so as to provide a minimum axial clearance between them before the bearing surfaces come into contact, which axial clearance reduces to zero or almost zero when the bearing surfaces come into abutment. These transverse abutting surfaces can also, conventionally, absorb the makeup torque, but EP-A-0 454 147 does not disclose a high makeup torque threaded connection.
In the present invention, when connection makeup is complete, the diametrical interference between the thread crests of each of the two male and female threads and the mating thread roots is preferably negative or zero.
Very preferably, when the two flanks of the male thread come into contact with their mating female threads during makeup, a radial clearance of at least 0.15 mm subsists between the crests and roots of the mated threads.
In a preferred variation, the diametrical interference between the thread crests of one only of the two male or female threads and the thread roots of the mated thread is positive when makeup is complete.
Preferably, to obtain an effective axial interference fit of the threads by a wedge effect, the angle xcex4 between the load flank and the stabbing flank of the male or female threads is less than or equal to 20xc2x0.
Highly preferably, it is in the range 7xc2x0 to 20xc2x0 and more particularly, close to 10xc2x0.
Preferably again, the thread crests of each of the male and female threads overhang the thread roots of the same thread on the load flank side, the angle xcex1 between the load flank and the normal to the connection axis thus being negative and having a value in the range 0 to xe2x88x9215xc2x0.
Advantageously, if too severe a wedge effect causing too high a slope dT/dN of the curve of the makeup torque as a function of the number of turns is to be attenuated, at least one of the male and/or female threads comprises a groove opening into the thread crest over all or a portion of the length of the thread or threads.
Such a groove increases the flexibility of the thread and somewhat reduces the axial interference fit forces and reduces the more these forces as the groove has a substantial depth and width. This results in a substantial reduction in the slope dT/dN of the curve of the makeup torque as a function of the number of turns at the expense of a slight reduction in the maximum makeup torque. Thus the combination of the two characteristics, maximum makeup torque and slope dT/dN, can be optimised.
French patent FR-A-2 408 061 describes threaded connections with trapezoidal threads in which one of the threads carries a type of groove opening into the thread crest.
However, that groove is closely associated with structures of the thread flanks producing a self-locking connection, i.e., resisting breakout: to this end, the inclination of the thread flanks with the groove is different from that of the thread flanks with no groove and is such that the width of the groove at its opening reduces during makeup under the bending forces resulting from the differences in orientation of the mating thread flanks.
This document does not disclose the function of the means of the connection of the invention and is not applicable to threads with the characteristics of the invention, in particular a female thread form which mates with the male threads.
Preferably, in accordance with the invention, the groove depth is at most equal to the thread depth and the groove width at its opening into the thread crest is at most ⅔ of the thread width, the thread depth being the radial distance measured perpendicular to the connection axis between the taper enveloping the thread crests and that of the thread roots and the thread width being measured parallel to the connection axis at the thread mid-depth.
Preferably again, the groove has, in a longitudinal axial plane, a U profile with arms which may or may not be parallel, or in the shape of a V with a rounded base.
Very preferably, the rounded base of the groove has a radius of at least 0.2 mm to prevent stress concentration in the groove base.
Preferably again, each of the male and female elements comprises at least one sealing surface, the orientation of each male sealing surface being substantially longitudinal and radially interfering with a mating female sealing surface at the end of the connection makeup so as to seal the connection.
Preferably again, each of the male and female elements comprises at least one substantially transverse bearing surface, at least one abutting male bearing surface coming into abutment against a female bearing surface at the end of the connection makeup to precisely position the sealing surfaces and thus define their interference.
Such bearing surfaces do not, however, act to define the position at the end of connection makeup when the stabbing flanks of the threads come into contact.